This invention pertains to the making of ammunition cartridges for rifles such as those used by hunters, military, and competitive shooters.
Rifle chamber and ammunition cartridge designations are standardized by the Sporting Arms and Ammunition Manufacturer's Institute (SAAMI). ANSI maintains a corresponding standard Z299.4. These standards define, among other things, the physical dimensions of the cartridge and chamber for each cartridge designation by way of a mechanical drawing specifying the dimensions and tolerances for each feature. These dimensions and tolerances dictate how the cartridge will fit into the chamber, take into account changes in the cartridge dimensions during firing, and also account for normal manufacturing variation to ensure that all commercial ammunition will function in all commercial rifles.
Allowable variations are small but they can have a significant effect on accuracy because they may alter the way a bullet enters the barrel; which affects how it leaves the barrel; which affects downrange accuracy. One of the motivations for hand loading ammunition is to take advantage of the ability to adjust the final dimensions of the cartridge to closely match the chamber of a particular rifle and to also decrease the variation from cartridge to cartridge; thereby increasing accuracy and consistency. Among other things, careful loaders are concerned with the concentricity of the bullet to the bore of the barrel.
Any lack of concentricity between the bullet and the bore will cause the center of mass of the bullet to rotate about the central axis of the bore as the bullet moves down the barrel. Once the bullet is in flight the center of mass will continue to rotate about the center of rotation; which results in wobbling during flight; which has an appreciable deleterious effect on downrange accuracy. Shooters have been aware of this for a long time, so careful hand loaders have long used a variety of tools and techniques to reform the cartridge case in an effort to eliminate any lack of concentricity. However, those tools and techniques were developed based on a theory of operation that is only properly applied to a subset of rifle cartridge types.
Ammunition cartridges are assembled from a case, a primer, powder, and a bullet and are put into several broad classes based on the type of case that is used: rimmed, rimless, and belted being the most common types. Each of these three types of cases use a different physical feature on the case to locate the case inside the chamber, which is commonly called ‘headspacing’ but within the SAAMI specification is called ‘breeching’. Rimmed and belted cases are breeched by (“headspace off of” is the common terminology) the rim or belt, both features being located at the head of the case (end opposite the bullet). Rimless cases are breeched by (“headspace off of”) the shoulder, the conical transition between the larger cylindrical body of the case; which holds the powder; and the smaller cylindrical neck; which holds the bullet. This is a difference that the prior art has not addressed. All of the different reloading dies, overall-length-gauges, bullet comparators, other tools, and the techniques for using them that are contained in the prior art and commercially available make no differentiation between these different case types, essentially treating all of them as if they were of the rimmed type; where all critical dimensions are referenced from the head of the case and alignment is controlled by adjustments to the body and neck of the case. However, for rimless shoulder breeching cartridges critical dimensions are properly referenced to the case shoulder and it is the case shoulder that should be used to align the bullet.
This is a critical distinction that is being ignored by the existing technology; so I have designed a set of tools for loading cartridges that are similar to existing tools but are designed specifically for rimless shoulder breeching rifle cartridges. These include tools for measuring the chamber and ammunition properly, a case trimmer, a neck sizing die, and a bullet seating die, each of which is the subject of a separate invention disclosure. This disclosure is for a tool similar to a case trimmer.